As working modes of a lawn mower, basically there are the three working modes of mulching work, bagging work, and discharge work.
Mulching work is a working mode in which two cutter blades are provided in a cutter housing, grass is cut with these cutter blades, and finely cut grass cuttings are dispersed on the lawn surface after cutting so that they are inconspicuous.
Bagging work is a working mode in which one cutter blade is provided in the cutter housing, grass is cut with this cutter blade, and the cut grass is received in a grass bag attached to the back of the cutter housing.
Discharge work is a working mode in which one cutter blade is provided in the cutter housing, grass is cut with this cutter blade, and the cut grass is discharged onto the mown lawn surface from the rear of the cutter housing.
A lawn mower made to perform mulching work is proposed in for example Japanese Utility Model Registration No. 2516509.
The lawn mower disclosed in this Japanese Utility Model Registration No. 2516509 will be described below on the basis of FIG. 13 and FIG. 14 hereof.
The lawn mower 240 of related art shown in FIG. 13 has a motive power source 242 mounted on the top of a cutter housing 241. An upper cutter blade 245 for mulching work only and a lower cutter blade 244 for bagging work and discharge work are attached one above the other to the output shaft 243 of the motive power source 242. The lower cutter blade 244 and the upper cutter blade 245 are rotated inside the cutter housing 241 and cut grass.
As shown in FIG. 14, in plan view the upper cutter blade 245 is parallel with and stacked on the lower cutter blade 244. However, the upper cutter blade 245 is positioned close to air-lifting parts 246, 246 of the lower cutter blade 244. Consequently, there has been the shortcoming that as a result of a swirl current and lift current creation function of the air-lifting parts 246, 246, which are for creating a carrying draft in the cutter housing 241, being obstructed, as shown in FIG. 13, the airflows in the vicinities of the air-lifting parts 246, 246 are disrupted and there is a resulting increase in noise.
That is, as shown in FIG. 13, because the air-lifting parts 246, 246 of the lower cutter blade 244 extend above the blade edges of the upper cutter blade 245, the path of the carrying draft is narrowed by the air-lifting parts 246, 246, and the swirl current and lift current creation function is impaired.
Also, as shown in FIG. 14, the phases of the blade edges 247, 247 of the lower cutter blade 244 and the blade edges 248, 248 of the upper cutter blade 245 overlap. Because of this, it sometimes happens that when grass is cut by the upper cutter blade 245, this grass is pushed over (made to lean) in front of the blade edges 248, 248, and this leaned grass is to be cut by the blade edges 247, 247 of the lower cutter blade 244, and the grass cannot be cut with certainty by the lower cutter blade 244.
Accordingly, a lawn mower has been awaited with which it is possible to avoid interference between the lower cutter blade and the upper cutter blade and reduce the noise of these cutter blades.